Chapter 7 PowerPoint Flashcards
What is equilibrium?
Equilibrium: state achieved if the resultant force and net torque (moment) acting upon an object are equal to zero
Equilibrium can be either ____ or ____?
Equilibrium can be either linear or rotational
What is linear/translational equilibrium?
Linear or translational equilibrium: achieved when the net of the external forces acting upon a system is equal to zero
e.g., a tug-of-war
What is rotational equilibrium?
Rotational equilibrium: achieved when the net of the external torques (moments) acting upon a system is equal to zero
What is static equilibrium?
Defined as a system in linear and rotational equilibrium and possessing 0 linear or rotational velocity
The object is completely at rest
What three conditions must occur for static equilibrium to be acquired?
3 conditions occur for a static equilibrium state to be acquired
∑▒〖𝐹𝑦=0〗
∑▒〖𝐹𝑥=0〗
∑▒〖𝑇=0〗
Sums of all Vertical, Horizontal, and Torques are zero
What is dynamic equilibrium?
Don’t assume that because of 0 net force or Torque the object isn’t moving
Riding in an Elevator? Or Riding in a Car or Airplane?
Dynamic Equilibrium
A system in motion but it is experiencing no change in velocity or direction
What equations must be satisfied for dynamic equilibrium?
Dynamic equilibrium requires satisfying all of the following equations:
ΣFx - mαx = 0 ΣFY - mαY = 0 ΣFZ - mαZ = 0
ΣTx - Ixα x = 0 ΣTY - IYαY = 0 ΣTZ - IZαZ = 0
What is ΣF?
ΣF = the sum of the forces in a given direction
What is ma?
ma = the product of the body’s mass and acceleration
What is ΣT?
ΣT = the sum of the torques about the given axis
What is Iα?
Iα = the product of the body’s rotational inertia and angular acceleration about the given axis
What is stability?
Stability: the resistance of an object to having its equilibrium disturbed
The more stable the object, the more resistance it has against forces or torques that attempt to disrupt its equilibrium.
What is balance?
Balance: the ability to control the current state of equilibrium, and it implies conscious effort and coordination
Stability is more of a ____ term, whereas balance is a ____ reference?
Stability is more of a mechanical term, whereas balance is a neuromuscular reference
What is stable equilibrium?
Defined as a state in which a large force or torque is required to disrupt an object’s current position.
Note the Pyramid example
A lower center of gravity requires a longer torque to be applied to push the object over.
Coaching tip for most sports.
What is restorative force?
Restorative Force
Uses gravity to return the object to the original position
Gravity is a torque on the object.
Line of gravity still on the original side of the axis of rotation
What is a disruptive force?
Disruptive force
Force applied caused the line of gravity to move on the opposite side of the axis of rotation.
The more torque required to overcome the ____ ____, the more ____ the object is considered to be?
The more torque required to overcome the restorative torque, the more stable the object is considered to be.
What is unstable equilibrium?
State in which very little force or torque is required to disrupt an objects current position
Even a small force applied creates enough torque for the pyramid to fall over.
COG is not lifted at all, it only falls.
Pencil example
What is neutral equilibrium?
State in which an object has no tendency to fall in one direction or another.
Think of a Sphere
The only way to disrupt neutral equilibrium is by tilting the surface
What is linear stability?
Linear Stability – the resistance of a body to having its linear equilibrium disrupted.
Increased stability often accompanied by decreased ____?
Increased stability often accompanied by decreased mobility
What is a major factor of an object that determines its stability?
Mass of Object – major factor
Greater force required to linearly accelerate a more massive object
Greater mass leads to increase friction force
What is rotational stability?
Rotational Stability – the resistance of a body to having its rotational or angular equilibrium disrupted
Major factor – mass of object
> inertia(mass) of object the more Torque is require to disrupt equilibrium.
Major factor – location of center of gravity relative to Base of support.
What is base of support?
The entire area formed by the objects points of contact with resistive surface providing counterforce.
Rotational stability is directly related to the location of the center of gravity.
COG in middle of base of support, then no force arm. Object stays in rotational equilibrium.
Rotational stability is directly related to the location of the ____ ____ ____?
Rotational stability is directly related to the location of the center of gravity.
How can rotational stability be increased?
Rotational Stability can be increased by:
Keeping COG in middle of Base of support
Lowering COG of object
Moving COG to one side of base of support as long as it’s the same side of the external force being applied.
Describe the keys of the base of support?
Size and shape of BOS affects the ease with which the line of gravity falls outside the boundaries of the base.
A wide base in the direction of line of action of external force
The key is to put as much distance between the line of gravity and the potential axis of rotation as possible.
Describe the relationship of rotational stability and mass?
Larger mass translates into a larger rotational inertia
Greater torque required to provide angular acceleration
Sumo wrestler will be stable both linearly and Rotationally as long as he maintains a wide stance and low center of gravity
Same is true for other sports
How is stability relative in the medical world?
PT/OT
Work with patients in need of restoring Balance, controlling equilibrium after injury or from a congenital condition.
Proprioceptors – body’s built in sensory system communicating with the brain
Reporting information on Joint position, velocity, muscle tension, rate of change in length.
Tons of equipment for Balance training
